The primary role of the immune system is to discriminate between self and non-self. Specific recognition of antigen is dependent on the ability of lymphocytes to generate a wide array of clonal receptors. The variable regions of the T cell antigen receptors (TCR) are encoded by separated gene segments which undergo rearrangement and joining during T cell development. The major source of T cell receptor diversity resides at the site of gene segment joining and random addition of junctional nucleotides. This latter process appears coincidentally with terminal deoxynucleotidyl transferase (TdT) activity, although the contribution of TdT has not yet been formally proven in vivo. Control of the T cell receptor repertoire is essential to the prevention of autoimmune disease. Although alpha beta T cells are subjected to selection in the thymus; the site of gamma delta T cell selection is still a subject of intense investigation. During murine fetal life, initial waves of gamma delta T cell precursors express limited junctional diversity and a selected germline repertoire. Adult dendritic epidermal cells (DEC) bear a similar germline TCR repertoire and also express little junctional diversity. In this study, we propose to create transgenic mice which express TdT at high levels during fetal life. Through sequence analysis of gamma delta TCR rearrangements in fetal thymocytes, we will test the influence of inappropriate TdT activity on the generation of N region diversity in vivo. Through examination of adult DEC cells, we will determine whether or not the virtually monomorphic T cell receptors found in this T cell subset is the result of positive selection, or simply due to their derivation from fetal thymocytes which similarly fail to exhibit junctional diversity. These studies should not only shed light on the mechanisms which control diversification of the T cell antigen receptor repertoire, but in the future will allow us to test the effect of abnormal diversification of the repertoire on the development of gamma beta T cells and in other mouse models of autoimmune disease.